The present invention concerns a method for equalizing a receive data block in a time-division multiple access communication system.
It also concerns a receiver implementing this method.
In a time-division multiple access (TDMA) communication system, received data blocks have to be equalized to allow for time differences in multipath media causing intersymbol interference.
FIG. 1 is a diagrammatic representation of a data block BD comprising start and end symbols SD and SF, two data sequences D, D', with respective lengths N.sub.D, N.sub.D, and a learning sequence SA inserted between the two data sequences. The sizing of this data block represents a compromise between:
service requirements, i.e. the number 2N of data symbols to be transmitted per block; PA1 the rate of variation of the transmission channel which imposes an upper limit N.sub.max on the number of data symbols in the block and lower and upper limits on the length of the learning sequence; and PA1 the required efficiency of the radio channel, which sets a lower limit on the ratio of the number of data symbols in the block to the total block length.
An equalizer is used to correct intersymbol interference in a receiver. To operate correctly, the equalizer must know the impulse response of the transmission channel. To this end known, special symbols are transmitted in the learning sequence. It is assumed that the data symbols transmitted are not known to the receiver. The learning sequence is chosen to suit the characteristics of the transmission channel and, in particular, its length. The learning sequence is usually made up of K precursor symbols, P reference symbols and possibly K "postcursor" symbols where K is the length of the channel defined as the number of symbol durations which is equivalent to the difference between the longest path and the shortest path in the channel and where P&gt;K.
The receiver uses a channel estimator device to establish the impulse response of the channel. It generates a replica of the learning sequence and correlates it with the corresponding received symbol sequence. The result of this correlation is a set of coefficients h.sub.i where i varies from 0 to L and L is the length of the channel, this set of coefficients being used to "teach" the equalizer. h.sub.0 represents the most direct path in the channel and all other coefficients represent longer paths which cause interference with the first.
It is essential to size the data sequences of a block in such a way as to be able to reconcile a sufficient or specified efficiency of the radio path, and a valid estimate of the impulse response of the channel for all data symbols, including those at both ends of the block and therefore farthest away from the learning sequence.
A first solution to the problem of intersymbol interference, at the receiving end, is to provide equalization without any device for tracking channel variations. If the received block is too long, the impulse response estimated at the center is no longer valid at the ends. This imposes the use of very short blocks, implying a small number of data symbols as compared with the number of learning symbols.
Another solution is to provide equalization in conjunction with continuous use of a tracking device. However, if the block received is short in comparison to the rate of variation of the channel, the impulse response estimated at the center remains valid at the ends. In this case the tracking device is used when it is not necessary to use it, leading to excessive power consumption.